Chapter XII
. They may be used to transmit wireless messages simply by connecting to a spark-gap and a key.
Spark coils which are especially made for wireless telegraphy will usually send farther than an ordinary spark coil used for experimental purposes.
[Illustration: Fig. 221.—A Wireless Spark Coil.]
A good one-inch coil costs from $4.50 to $5.00 and will send from three to five miles if used with a fair aerial.
A spark coil requires considerable current for its successful operation and will give the best results if operated on storage cells, dry cells, or bichromate cells. If dry cells are used, it is a good plan to connect them in series multiple as shown in Figure 69.
Spark-gaps may be made by mounting two double binding-posts on a wooden base as shown in Figure 222.
Zinc possesses some peculiar property which makes it very efficient for a spark-gap, and for this reason the electrodes of a spark-gap are usually zinc.
[Illustration: Fig. 222.—Small Spark Gaps.]
The figure shows two different forms of electrodes. In one, they are made of zinc rods and provided with “electrose” handles. In the other gap, the zinc electrodes are in the shape of "tips" fitted on the ends of two short brass rods.
A one-inch spark coil will give very good results by connecting the spark-gap directly across the secondary of the coils. The aerial is connected to one side of the gap and the ground to the other.
The transmitter may be "tuned" and the range sometimes increased by using a condenser and a helix.
A condenser is most easily made by coating the inside and outside of a test-tube with tinfoil so as to form a miniature Leyden jar. The end of the tube is closed with a cork through which passes a brass rod connecting to the inner coating of tinfoil.
[Illustration: Fig. 223.—Diagram showing how to connect a Simple Transmitter.]
If such a condenser is connected directly across the spark-gap, the spark will become very white and crackling.
Several tubes may be arranged in a rack as shown in Figure 225.
A helix consists of a spiral of brass ribbon set in a wooden frame. The two strips composing the frame are each nine inches long. The spiral consists of eight turns of brass ribbon, three-eighths of an inch wide, set in saw-cuts made in the frame. A binding-post is connected to the outside end of the ribbon.
Figure 228 shows how to connect a helix and a condenser to a coil and a spark-gap.
The two clips are made by bending a strip of sheet brass and connecting a piece of flexible wire to one end.
[Illustration: Fig. 224.—A Test-Tube Leyden Jar.]
In large stations, the best position for the clips is found by placing a "hot-wire ammeter" in the aerial circuit and then moving the clips until the meter shows the highest reading.
The young experimenter will have to tune his set by moving the helix clips about until the best results are obtained in sending.
[Illustration: Fig. 225.—Eight Test-Tube Leyden Jars mounted in a Wooden Rack.]
If the spark coil is a good one and capable of giving a good hot spark, it may be possible to tell when the set is in proper tune by placing a small miniature tungsten lamp in series with the aerial and changing the clips, the condenser, and the length of the spark-gap until the lamp lights the brightest.
An _oscillation transformer_ is sometimes used to replace an ordinary helix when it is desirable to tune a station very closely so that its messages shall not be liable to be confused with those of another station when both are working at the same time.
[Illustration: Fig. 226.—A Helix and Clip.]
An oscillation transformer consists of two helixes arranged so that one acts as a primary and the other as a secondary. An oscillation helix may be made by making two sets of helix frames similar to that in Secondary Figure 226.
[Illustration: Fig. 227.—An Oscillation Transformer.]
The primary should be provided with eight turns of brass ribbon and the secondary with twelve. The primary supports a stiff brass rod upon which the secondary is mounted. The secondary should slide up and down on the rod but move very stiffly so that it will stay where it is put.
[Illustration: AN OSCILLATION HELIX.]
[Illustration: AN OSCILLATION CONDENSER.]
An ordinary double-throw, double-pole knife switch having a porcelain base will make a very good aerial switch in a small station. It is used to connect the aerial and ground to either the transmitting or receiving apparatus at will. Such a switch is shown in Figure 230.
[Illustration: Fig 228.—Circuit showing how to connect a Helix and a Condenser.]
The aerial should be connected to the post _A_ and the ground to _B_. The posts _E_ and _F_ lead to the transmitter, and _C_ and _D_ to the receptor, or vice-versa according to which is the more convenient from the location of the apparatus on the table or operating bench.
A suitable table should be arranged to place the wireless instruments upon so that they may be permanently connected together.
[Illustration: Fig 229.—Circuit showing how to connect an Oscillation Transformer and a Condenser.]
The Continental Code is the one usually employed in wireless telegraphy. It differs slightly from Morse as it contains no space letters. It will be found easy to learn and somewhat easier to handle than Morse.
[Illustration: Fig 230.—An Aerial Switch.]
Two or three months’ steady practice with a chum should enable the young experimenter to become a very fair wireless telegraph operator. Then by listening for some of the high power wireless stations which send out the press news to ships at sea during the evening it should be possible to become very proficient. The press news is sent more slowly than ordinary commercial wireless messages, and is therefore easy to read and a good starting point for the beginner learning to read.
[Illustration: Fig 231.—A Complete Wiring Diagram for both the Transmitter and the Receptor.]
[Illustration: Fig. 232.—The Continental Alphabet.]
*A Coherer Outfit*
*A Coherer Outfit* is usually capable of only receiving messages coming from a distance of under one mile. In spite of this fact, however, it is an exceedingly interesting apparatus to construct and experiment with, and for this reason is found fully described below.
A coherer set will ring a bell or work a sounder for short distances and therefore is the best sort of an arrangement for demonstrating the workings of your wireless apparatus to your friends.
The first thing that you need for a coherer is a pair of double binding-posts. Mount these about an inch and three-quarters apart on a wooden base, six inches long and four inches wide as shown in Figure 233.
[Illustration: Fig. 233.—A Coherer and a Decoherer.]
Get a piece of glass tubing about an inch and one-half long and about one-eighth of an inch inside diameter. You will also need some brass rod which will just slide into the tube tightly. Cut off two pieces of the brass rod each one and three-quarters inches long and slip these through the upper holes in the binding-posts and into the glass tube as shown in Figure 234. Before putting the second rod in place, however, you must put some nickel and silver filings in the tube, so that when the rods are pushed almost together, with only a distance of about one-sixteenth of an inch between them, the filings will about half fill the space.
The filings must be very carefully prepared, and in order to make them, first use a coarse-grained file on the edge of a five-cent piece. Do not use the fine dust and powder, but only the fairly coarse filings. Mix a few silver filings from a ten-cent piece with the nickel in such proportion that the mixture is 90% nickel and 10% silver.
[Illustration: Fig. 234.—Details of the Coherer.]
You will have to experiment considerably to find out just the right amount of filings to place in the tube, and how far apart to place the brass rods or plugs.
Remove the gong from an old electric bell and mount the bell on the base as shown in Figure 233. It should be in such a position that the bell hammer will touch the coherer very lightly when the bell is ringing.
The two binding-posts, tube rods, and filings constitute the _coherer_. The bell is the _decoherer_.
The next thing required in order to complete the apparatus is a relay. You may use the relay described in